The Claessen lab studies morphogenesis and phenotypic heterogeneity in filamentous actinomycetes, which are prolific antibiotic producers. We pursue several research projects that span areas of multicellular growth and development, stress-adaptation and microbial evolution. By using multidisciplinary approaches, we aim to tackle fundamental questions, and use the generated knowledge to improve industrial exploitation of these organisms. Dennis Claessen is also the current head of the cluster Microbial Sciences.

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The cell wall is a shape-defining structure that envelopes almost all bacteria and protects them from environmental stresses. Bacteria can be forced to grow without a cell wall under certain conditions that interfere with cell wall synthesis, yielding wall-less cells known as L-forms. We have shown that several species of filamentous actinomycetes have a natural ability to generate wall-deficient cells in response to hyperosmotic stress. This wall-deficient state is transient and these cells are able to switch to the normal mycelial mode of growth. Interestingly, prolonged exposure to hyperosmotic stress yields variants that are able to proliferate indefinitely without their cell wall, similarly to L-forms. This indicates that the formation of wall-deficient cells in actinomycetes serves as an adaptation to osmotic stress. We are currently studying this remarkable response in more detail, and exploit the unique properties of these cells to obtain fundamental new insight into the plasticity of bacterial life.

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